Since I added 2 GB of RAM to my desktop PC, it has started locking up on me. I can see no obvious trigger. It can happen when I'm browsing the internet, reading e-mail, performing a virus scan or working on an excel spreadsheet. Does anyone know what causes this and how I can fix it?

Could be a defective module, modules not 100% compatible with your motherboard or memory timings, or modules that do not perform well together if you are using the new ones with existing old ones. First try simply removing and reseating the modules and verify the slots are absolutely clean and dust-free with an electronics duster. Then, if the problem still exists, isolate whether it is a hardware issue or a software issue that is now exacerbated by the change in memory configuration by running a memory diagnostic such as Memtest86+ from a bootable floppy or usb drive for several passes to thoroughly exercise all the memory. Or, remove all but one or two modules and run the computer as usual for hours or days swapping the modules in logical combinations to determine which pairings might be problematic. The memory timings in the BIOS, if configurable, could also be backed off to see if it has any impact and also may allow any mismatched pairs to work better together.

There is also the possibility, although slight, that if anti-static precautions were not observed when the modules were installed, a module or the motherboard was damaged.